The present disclosure is generally related to the field of medical imaging, and more particularly the field of radiography. Embodiments of the present invention relate to the field of the deploying anti-scattering grids used to improve radiographic images by filtering the photons scattered by the organ under study, and keeping only the photons emitted by the source. Embodiments of the present invention can be utilised within the scope of mammography, and more particularly within the scope of breast tomosynthesis, which takes a series of frames at different angles to produce a 3D image of the object being studied.
Anti-scattering grids are used widely in radiography devices to eliminate the effects generated by the parasitic scattering of some photons taking place in the organ studied by said devices. These grids filter photons scattered by the organ being studied and mainly keep only photons actually originating from the radiation source of the radiography device, thus improving the contrast of the obtained images.
In FIG. 1, a conventional use of an anti-scattering grid 2 is illustrated. The grid 2 is placed between a body part to be studied of a patient P who is irradiated by a radiation source 1, and a radiation detector 3 comprising a network of sensors 31 (illustrated in FIG. 3) distributed periodically with a period pd (later called the pitch of the detector).
The assembly made up of the grid and the detector is positioned according to a plane perpendicular to the plane PT of the torso of the patient. By way of non-limiting example, illustrated in FIG. 1, the patient P can be standing and the grid and the detector are then on a horizontal plane, the walls of the grid and of the detector in contact with the patient being tangential to the plane PT,
The grid 2 is generally constituted by alternating radio-opaque and radio-transparent strips 21, the axes of strips 21 being parallel and distributed periodically, with a pitch pg between two radio-opaque strips so that the scattered photons are absorbed by the radio-opaque strips and the photons coming directly from the source 1 are transmitted to the detector 3 of the radiography device. In some embodiments, known as “focused grids,” the plane of the strips perpendicular to the plane of the detector is slanted to match the beam divergence from the focal spot to the detector at a specific source-to-detector distance.
One drawback of using such grids is that an image of the radio-adsorbing strips of the grid appears on the detector. Also, alternating strips can cause interference figures, or moiré effect, on the detector and deteriorate the quality and legibility of the image obtained.
For erasing the image of the radio-adsorbing strips, commonly termed “erasing the grid lines” and similar phrases, a solution known in mammography and schematically illustrated in FIGS. 1 and 2A is used, consisting of animating the grid 2 by any vibration movement perpendicular to the direction according to which the strips 21 extend.
In terms of breast tomosynthesis, acquiring a 3D image of the object means acquiring a series of images of the object according to different relative angular positions between the source, the detector, and the body part under study. Since it is desired to reconstruct tomographic images of the whole breast, including of areas as close as possible to the chest wall it is required that all successive images (known as “frames”) acquired during the tomographic sweep contain information relative to all points of the volume of the breast. By simple geometric construction, the natural consequence is that most images should be taken with the source in the plane of the chest wall. Therefore the movement of the source between images should reside in this plane, and consequently the grid focusing line as well. For this to occur, the radiation source 1 is pivoted about an axis Y-Y, illustrated in FIG. 2B, perpendicular to the plane of the torso of the patient P.
In standard static imaging the orientation of the grid is generally with the grid strips and the focusing line perpendicular to the chest wall. In breast tomosynthesis the grid strips have to be oriented parallel to the chest wall, i.e., pivoted 90° relative to the usual orientation.
For reasons associated with bulk, and limited space between the chest wall of the patient and the wall of the detector/grid assembly, as illustrated in FIG. 3, it then becomes difficult to execute displacement of the grid 2 perpendicularly to the direction of the strips.
A further problem with this displacement, if periodic in nature, is the creation of shadowing and decreased contrast caused by the return points of the radio-adsorbing grid strips. More particularly, return points arise as a function of the amplitude and frequency of the movement of the grid and are defined as the point at which the direction of travel alters from one direction to a second (typically opposite) direction. Return time is defined as the time centered on the return point in which grid movement slows down, stops, and resumes in the opposite direction with a comparable speed.
Current patterns of grid movement which use a fixed frequency or amplitude for grid movement increase the risk of making shadows from the grid strip at the return points visible in the images. A solution would be to reduce the return time to a degree where its impact becomes negligible on the strip visibility. However, by way of an illustrative example, for a 2 mm amplitude periodic movement and 50 ms exposure time this would require accelerations as high as 30 g, which cannot be achieved at acceptable bulk and cost with currently available technologies.
In effect, in reference to FIG. 3, the detector 3 and the anti-scattering grid 2 are located under a cover 4 likewise supporting the breast of the patient throughout examination. Legislation imposes that the distance between the chest wall of the patient and the closest sensors 31 of the detector 3 be less than 5 mm.
This interval must also comprise the thickness of the cover 4 and the inactive edge of the grid 2 where no strips are present. Given these elements, the space remaining for the grid 2 to move is less than 2 mm.
Since the movement required to erase the image of the grid is of the order of 10 mm, due to spacing constraints this movement cannot occur along an axis perpendicular to the plane of the torso PT of the patient.
Solutions adopted in breast tomosynthesis in the prior art to eliminate the image of the grid on the detector propose adapting the pitch of the grid pg to the pitch of the detector pd, so that the pitch of the grid pg is, for example, equal to a multiple of the pitch pd of the detector.
Another solution presented in document FR 2,939,019 consists of adapting the pitch of the grid to the Nyquist frequency of the detector then digitally filtering the image of the grid on the detector.
As outlined above, none of these solutions gives a completely satisfactory result. In particular, even if the grid is no longer visible on the frame, a moiré effect remains, linked to interference between the strips of the grid and the network of sensors of the detector.
What is needed, therefore, is an apparatus and process for making an image with a grid having strips parallel to the chest wall of the patient P, while at the same time avoiding any residual image of the grid strips or resulting moiré patterns in the image.